Video signal level control



May 10, 1949. MAYLE 2,469,606

VIDEO SIGNAL LEVEL CONTROL Filed Oct. 25, 1945 ALTERNATING CURRENTsouacs I Q 1 I I I/e [6 g s: I r! 5 INVENTOR LOUIS F. MAYLE VlDEO SIGNALSOURCE ATTORNEY Patented May 10, 1949 UNITED STATES PATENT OFFICE,

VIDEO SIGNAL LEVEL CONTROL I Louis- F. Mayle,Fort Wayne, Ind., asslgnor,by mesne assignments, to Farnsworth Research Corporation, a corporationof Indiana Application October 25, 1945, Serial No. 624,340

Claims. (Cl. 332-38) rent component, is of a relatively low frequencyand represents the average light value of the television subject. Bothcomponents are necessary for the faithful reproduction of a subject. Inone type of video signal generating device in common use, only thealternating current component of the video signals is produced. The socalled direct current component is developed separately and combinedwith the alternating current component for modulating a carrier currentwave. In another type of video signal generating device, there areproduced both the alternating and the direct current components. Whenusing the latter type of device, it is desirable to employ amplifiersfor the video signals which are capable of providing the requiredamplification of both signal components.

In any case, it is customary to modulate a power amplifier which isseparately excited by carrier current wave energy with a video signalconsisting of both components. In such cases, grid modulation of thepower amplifier is most generally used by impressing the video signalsupon the grid circuit of the amplifier. Such amplifiers, for reasons ofeflecting eificient operation, customarily are arranged for class Coperation. This type of amplifier is provided with a negative grid biasfor this purpose. Frequently the video signals which are used tomodulate a classC power amplifier have a direct current component whichvaries over a considerable range. It has been found that, when thisdirect current component has a relatively high negative value, the poweramplifier, by reason of its negative grid bias, may be cut oil. inresponse to such' signals with the result that the modulation of thecarrier current wave will not correspond accurately to the shade valuesof the television subject.

It, therefore, is an object of this invention to provide a signal levelcontrol system of such a character to permit the use of intelligencesignals having a direct current component which varies over aconsiderable range for modulating a power light and I One of thesecomponents is of v 2 amplifier without biasing the amplifier to cut-oflat one extreme value of the direct current component range of variation.

Another object of the invention is to provide a video signal levelcontrol system wherein the magnitude of the space current for a videosignal vacuum tube amplifier is varied under the control of the directcurrent component of the video signal derived from the output circuit ofthe vacuum tube amplifier.

In accordance with the invention, broadly, the signal level controlsystem is provided with a vacuum tube. amplifier having input and outputcircuits. The input circuit is coupled to a source of. intelligencesignals which may include both the alternating current and the directcurrent components. Unidirectional space current is supplied to theoutput circuit of the signal amplifier by a grid controlled vacuum tuberectifier, the input circuit of which is coupled to a source ofalternating current. The output circuit of the signal amplifier iscoupled to the control grid circuit of the rectifier so that theunidirectional current developed in the output circuit of the rectifieris varied in accordance with the direct current component of theamplified intelligence signals. By this means the level of these signalsmay be controlled so as not to exceed predetermined limits. Autilization circuit for the controlled signals is coupled to the outputcircuit of the signal amplifier.

More specifically, in accordance with a preferred embodiment of theinvention for use in a television system, amplified video signalsdeveloped in the output circuit of the vacuum tube amplifier are derivedfrom a cathode follower circuit connected to this tube. The rectifierfor supplying space current to the vacuum tube amplifier comprises apair of grid controlled vacuum tubes connected in a manner to eflect afull wave rectification of the alternating current. Also, theutilization circuit coupled to the cathode "follower video signalamplifier tube includes a class C power amplifier arranged for gridmodulation by the amplified video signals.

For a better understanding of the invention, together with other andfurther objects thereof, reference is had to the following description,taken in connection with the accompanying drawings, and its scope willbe pointed out in the appended claims.

The single figure of the accompanying drawing is a schematic'circuitrepresentation of a video signal level control system embodying thisinvention.

diagrammatically a video signal source I which may be any desired typeof video signal generator wherein there are produced in the outputcircuit video signals including the alternating current and the directcurrent components. For example, if an image analyzing tube of thecharge storage type such as an iconoscope is employed, the directcurrent component of the video signals may be separately generated andcombined with the alternating current component for the development of acomplete video signal in the output circuit of the source I. In the casewhere an image analyzing tube of the dissector type-is employed, bothcomponents of the video signals are present in the output circuitthereof which may, if desired, constitute the output circuit of thesource I. A video signal signal amplifier capable of amplifying bothcomponents of the video signals is conductively coupled to the outputcircuit of the source I. The amplifier may comprise a vacuum tube 2having its control grid coupled directly to one of the output circuitterminals of the video signal source. The tube 2 is arranged in acathode follower circuit wherein a load impedance device such as aresistor 3 is connected between the cathode and the negative terminal ofa source of unidirectional space current. The anode of the amplifiertube 2 is connected directly to the grounded terminal of theunidirectional current source.

The source of unidirectional current for the video signal amplifier tubecomprises a source of alternating current 4 and a rectifier forconverting the alternating current into unidirectional current. In theillustrative embodiment of the invention this rectifier includes a pairof grid controlled vacuum tubes and 6. The alternating current source iscoupled to the anodes of the rectifier tubes by a transformer 1comprising a primary winding 8 and a secondary winding 9. The cathodesof the rectifier tubes are connected to ground. A filter l| comprising aseries inductance element such as a coil I2 and a pair of shuntcapacitance elements such as the condensers l3 and I4 is connectedbetween the midpoint of the transformer secondary winding 9 and ground.The output terminals of the rectifier are connected to the terminals ofa load resistor l5. It is seen, therefore, that the positive terminal ofthe unidirectional current source is the grounded end of the resistor l5and the negative terminal is the end of the resistor I5 which isconnected to the cathode follower resistor 3.

A bleeder resistor I6 is connected in parallel with the space dischargepath of the video signal amplifier tube 2. The bleeder resistor isprovided with a tap I! which is conductively coupled to the controlgrids of the rectifier tubes 5 and 6. A bypass condenser I8 is connectedbetween the control grids of the rectifier tubes and ground.

The signal voltages developed in the output circuit of the video signalamplifier tube 2 are employed to effect grid modulation of a poweramplifier. The power amplifier comprises a pair of vacuum tubes l9 and2|, each including at least a cathode, an anode and a control grid. Thecathodes of these tubes are connected to ground. The control grids ofthe power amplifier tubes are connected respectively to the terminals ofan inductance coil 22. Another coil 23 is arranged for inductivecoupling to the grid coil 22 and is connected at its terminals to asource of a carrier current wave 24. The cathode of the video amplifiertube 2 is conductively coupled to the midpoint of the power amplifiergrid coil 22. A parallel resonant circuit 25 comprising an inductancecoil 26 and acondenser 21 is tuned to the frequency of the carriercurrent wave and is connected between the anodes of the respective poweramplifier tubes l9 and 2|. Space current for these tubes is suppliedfrom a source of unidirectional current such as a battery 28, thepositive terminal of which is connected to the midpoint of theinductance coil 26 and the negative terminal of which is connected toground. An output inductance coil 29 is inductively coupled to the coil26 and its terminals are connected to a load circuit 3|.

Before describing in detail the operation of the video signal levelcontrol system in accordance with this invention, a brief considerationwill be given to the operation of conventional apparatus wherein videosignals are employed to grid modulate a power amplifier arranged forclass C operation with no provision being made for controlling the levelof the video signals. Assume that the video signals derived from thesource l are impressed upon the control grid circuit of the amplifiertube 2 in such polarity that the signal representative of picture whiteis negative with respect to the signal representative of picture black.Since the tube 2 is arranged with a cathode follower circuit, theamplified video signals derived from the cathode thereof are in phase orhave a similar polarity to the video signals impressed upon the controlgrid. Also, assume that the amplified video signal voltagerepresentative of an elemental area of the television subject of normalbrightness is of such magnitude that the negative signal voltage derivedfrom the oathode of the tube 2 relative to ground effects a minimum ofspace current conduction in the power amplifier tubes l9 and 2|. A videosignal voltage derived from the cathode of the tube 2 representative ofdarker elemental areas of the subject will be of less negative magnitudeand, therefore, will eifect greater conduction of space current in thepower amplifier tubes.

However, an amplified video signal voltage derived from the cathode ofthe tube 2 which is representative of an elemental area of the subjectwhich is of a greater-than-normal brightness will be of greater negativemagnitude relative to ground. Consequently, because of the fixednegative biasing of the power amplifier control grids relative to theirassociated cathodes in order to obtain class C operation, such a signalvoltage will produce no response by the power amplifier tubes. Thecombination of bias and signal voltages impressed upon the grid circuitsof these tubes produces cut-off of space current therein. As a result,it may not be possible to modulate a carrier current wave with videosignals representative of the brightest elemental area of the subject.It, therefore, would be impossible under these circumstances tofaithfully reproduce the television subject on the screen of a receivingtube.

The video signal level control system in accordance with the inventionprevents the amplified video signal voltages derived from the amplifiertube 2 from exceeding the limiting voltages by which the power amplifiertubes l9 and 2| may be modulated. Assume that there is anincrease in theaverage brightness of a television subject. The direct current componentof the amplified video signals derived from the cathode of the tube 2will have a greater negative magnitude relative to ground. A highernegative voltage will rent in these tubes is decreased. Consequently,

the voltage drop produced in the rectifier load resistor 15 by theunidirectional current derived from the rectifier is decreased and alower space current producing voltage is impressed upon the amplifiertube 2. This lower voltage effects a reduction in the space current ofthe tube 2, thereby effecting a reduction in the voltage developed inthe cathode follower resistor 3.

It is seen, thus, that there is eflected by means oi the rectifiercontrol a compensation for any tendency for the negative magnitude ofthe amplified video signal voltages to exceed that which will efiect acomplete cutofi of space current in the power amplifier tubes i9 and 25.Accordingly, video signal voltages representative of all of theelemental areas of a television subject are efiec tive to suitablymodulate a carrier current wave, irrespective of changes in the averagebrilliance of the subject. Hence, the carrier wave which is radiated orotherwise transmitted to the receiver will be modulated by video signalsfrom which an accurate reproduction of the subject may be reproduced.

While there has been described what, at present, is considered thepreferred embodiment of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be. madetherein without departing from the invention, and therefore, it is aimedin the appended claims to cover all such changes and modifications asfall within the true spirit and scope of the invention.

What is claimed is:

l. A video signal level control system comprising, a video signal vacuumtube amplifier having input and output circuits, a source of videosignals coupled to the input circuit of said video signal amplifier, asource of alternating current, a rid controlled vacuum tube alternatingcurrent rectifier having an input circuit. coupled to said alternatingcurrent source and 'an output circuit coupled to the output circuit ofsaid video signal amplifier for supplying unidirectional space currentthereto, a coupling between the output circuit of said video signalamplifier and the control grid circuit of said. rectifier to control themagnitude of the unidirectional current supplied to said video signalamplifier in accordance with variations of the level of said videosignals, and a utilization circuit coupled to the output circuit of saidvideo signal amplifier.

2. A video signal level control system comprising, a video signalamplifier including a vacuum tube having input and output circuits, asource of video signals coupled to the input circuit of said videosignal amplifier tube, a load impedance device coupled to the outputcircuit of said video signal amplifier tube, a source of space currentfor the output circuit of said videosignal amplifier tube including agrid controlled alternating current rectifier tube having input andoutput circuits, 2. source or alternating current coupled to the inputcircuit of said rectifier tube, *a resistor connected in both the outputcircuits of said video signal amplifier and said rectifier tubes, autilization circuit coupled to the load impedance device of said videosignal amplifier tube, and a coupling between the output circuit or saidvideo-signal amplifier tube and the control grid of said rectifier tube.

3. A video signal level control system comprising, a video signalamplifier including a vacuum tube having a cathode, an anode and acontrol grid, a source'o! video signals coupled to the control grid ofsaid video signal amplifier tube, a load impedance device coupledbetween the cathode and the anode of said video signal amplifier tube, asource oi space current for said video signal amplifier tube including agrid controlled rectifier tube having a cathode, an anode and a controlgrid, a source of alternating current coupled between the anode and thecathode of said rectifier tube, a load resistor coupled to the anode ofsaid rectifier tube and also to the space discharge path of said videosignal amplifier tube, a power amplifier including a vacuum tube havingan input and an output circuit, a source of carrier current coupled tothe input circuit of said power amplifier tube, a load circuit coupledto the output circuit of said power amplifier tube, a, coupling betweenthe load impedance device of said video signal amplifier tube and theinput circuit of said power amplifier tube, and a coupling between thecathode of said video signal amplifier tube and the control grid of saidrectifier tube.

4. A video signal level control system comprising, a video signalamplifier including a vacuum tube having a cathode, an anode and acontrol grid, at source of video signals coupled between the controlgrid and anode of said video signal amplifier tube, a load impedancedevice connected to the cathode of said video signal amplifier tube, asource of space current for said video signal amplifier tube including apair of grid controlled rectifier tubes, each having a cathode, an anodeand a control grid, a source of alternating current coupled to theanodes of said rectifier tubes, a load resistor coupled between theanodes and the cathode of said rectifier tubes and also in series withthe space discharge path of said video signal amplifier tube and saidload impedance device, a power amplifier including a pair of vacuumtubes connected in push-pull relationship and having an input and anoutput circuit, a source of carrier current coupled to the input circuitof said power amplifier, a resonant circuit coupled in the outputcircuit of said power amplifier, a load circuit coupled to said resonantcircuit, a

coupling between the cathode of said video signal amplifier tube and theinput circuit of said power amplifier, a bleeder resistor connected tothe cathode of said video signal amplifier tube, and a coupling betweenthe control grids of said rectifier tubes and a point on said bleederresistor.

5. A video signal level control system comprising, a video signalamplifier including a vacuum tube-having a cathode, an anode and acontrol grid, a source of video signals including both the alternatingcurrent and the direct current components coupled to the control grid ofsaid video signal amplifier tube, a first load resistor connected .tothe cathode of said video signal amplifier tube, a source of spacecurrent for said video signal amplifier tube including a 7 pair of gridcontrolled rectifier tubes, each having a cathode, an anode and acontrol grid, a source of alternating current coupled to the anodes ofsaid rectifier tubes, a filter coupled between the anodes and thecathodes of said rectifier tubes, a second load resistor connectedacross the terminals of said filter and also in series with the spacedischarge path of said video signal amplifier tube and said first loadresistor, a power amplifier including a pair of vacuum tubes, eachhaving a cathode, an anode and a control grid, 9. source of carriercurrent coupled to the control grids of said power amplifier tubes, aresonant circuit coupled between the anodes of said power amplifiertubes, a load circuit inductively coupled to said resonant circuit, aconductive coupling between the cathode of said video signal amplifiertube and the control grids of said power amplifier tubes, a bleederresistor connected between the cathode and the anode of said videosignal amplifier tube, a conductive coupling between the control gridsof said recti- 8 fier tubes and a point on said bleeder resistor, and analternating current bypass condenser coupled between the control gridsand the.

cathodes of said rectifier tubes.

LOUIS 1''. mm.

REFERENCES CITED The following references are of recordin the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,173,496 Schlesinger 'Sept. 19,1939 2,239,776 Brunn Apr. 29, 1941 2,321,343 Wendt June 8, 19432,346,557 Conklin Apr. 11, 1944 FOREIGN PATENTS Number Country Date100,879 Australia Apr. 26, 1937 471,493 Great Britain Sept. 2,1937

